Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers or defibrillators for providing precisely controlled stimulation pulses to the heart. However, the appended claims are not intended to be limited to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac stimulator and the heart tissue which is to be stimulated. As is well known, the leads connecting such cardiac stimulators with the heart may be used for pacing, for sensing electrical signals produced by the heart, for defibrillation, or for a combination of those procedures in which case a single lead serves as a bidirectional pulse transmission link between the stimulator and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end an electrode designed to contact the endocardium, the tissue lining the inside of the heart. The lead further includes a proximal end having a connector pin adapted to be received by a mating socket in the pacemaker. A flexible, maybe coiled, conductor or plurality of conductors surrounded by an insulating tube or sheath typically couples the connector pin at the proximal end and the electrodes at or near the distal end.
The implantable cardiac stimulation leads with which the present invention is concerned may take the form, for example, of pace-makers capable of pacing and sensing in at least one chamber of the heart. Indeed, the present invention, may relate to a programmable dual chamber pacemaker wherein the basic configuration of the pacemaker, e.g. unipolar or bipolar, can be changed, including the grounding configuration and ground potentials used within the pacemaker.
Generally, a heart stimulator uses one or two flexible leads having one end connected to itself and the other end connected to electrodes placed in close proximity to the heart. These leads are used to stimulate or pace the heart. Also, these leads are used to sense the heart activity by picking up electrical signals from the heart.
In order to properly pace or sense or defibrillate, the cardiac stimulator device has to be able to deliver a stimulating pulse to the heart or to sense an electrical signal from the heart, and this requires that there be an electrical return path. If, within a given heart chamber, a unipolar lead is used containing a single conductor and the return path is the conductive body tissue and fluids. The return path is connected to the stimulator device by connecting the stimulator device's electrical common or ground to the stimulator's metal enclosure, typically referred to as the case or housing. The case or housing, in turn, makes contact with the body tissue and/or fluids.
An alternative solution to using a unipolar lead in a given heart chamber is to use a double lead/electrode in the heart chamber, known as a bipolar lead. In a bipolar lead, a second conductor is spiraled over and insulated from a first conductor along the length of the lead. At the distal end of the lead, one of the conductors is connected to a first electrode, referred to as the “tip” electrode, and the second conductor is connected to a second electrode, referred to as a “ring” electrode. The ring electrode is generally situated about 10 to 20 mm proximally from the tip electrode. The tip electrode is typically placed in contact with heart tissue, while the ring electrode is in electrical contact with the blood, and in some instances can also be making contact with heart tissue. Because body and heart tissue and fluids are conductive, the ring electrode of a bipolar lead, in contact with the body fluids or tissue, serves as an electrical return for both pacing and sensing.
As indicated, pacing or sensing using the pacer case or enclosure as part of the electrical return path is known as unipolar pacing or sensing. Pacing or sensing using the lead ring electrode and associated lead conductor as the electrical return path is known as bipolar pacing or sensing. There are numerous factors to consider when deciding whether unipolar or bipolar pacing and/or sensing should be used. Bipolar pacing has, in general, the advantage of requiring slightly less energy than unipolar pacing, and with regard to sensing, bipolar pacing impedance is usually greater than unipolar impedance. Further, bipolar sensing is less prone to far-field signals, crosstalk, and myopotential sensing than is unipolar sensing since its dipole is so much smaller. Crosstalk generally refers to a pacer mistakenly sensing a heart activity in one heart chamber immediately after the other chamber is paced. Bipolar sensing reduces crosstalk resulting from a pacing stimulus in the opposite chamber. Bipolar pacing is preferred if pectoral or diaphragmatic stimulation occurs.
Present day cardiac stimulation leads are required to have multiple conductors serving multiple electrodes. Each of the conductors is housed in lumina formed in the extruded insulation material. The insulation material may be composed of either silicone or polyurethane, or a combination thereof. Advances in technology have given rise to added complexity in the manufacture of pacing and defibrillation leads. Future designs of leads warrant simplicity in construction. This in turn gives rise to reduced touch time associated with the construction of leads and a reduction in costs associated from reduced steps of construction. Furthermore, over the past few years, there has been a substantial effort to reduce the diameter of endocardial pacing and defibrillation leads. The size of a lead body can facilitate placement of multiple leads through a single blood vessel and also minimize interference between the lead body and the tricuspid valve (applicable for leads implanted in the right ventricle or RV). Indeed, the ability to accommodate two leads in an 8 F introducer would be a remarkable advance. Indeed, this is the goal sought be the present invention.
Typical of the known prior art is U.S. Pat. No. 4,840,186 to Lekholm et al. In this instance, an implantable lead has conductors wound in a multi-pole helix. The conductors are individually insulated by a first insulating material and the insulated conductors are embedded in, and axially separated from each other by a second tube-formed insulating material. The inner opening of the tube receives a helically wound stylet guide coil which may also be a multi-pole conductor arrangement. The conductors may be helically wound, multi-filament wires. Preferably, according to the disclosure, all helically wound arrangements in the lead are wound in the same direction so that the lead is still when rotating it in one direction and flexible when rotating it the other way.
Another instance of the prior art is found in U.S. Pat. No. 4,559,951 to Dahl et al. Here, a catheter assembly designed for long term or short term implantation in an animal body includes a flexible tube of a biocompatible polymeric material in which plural electrical conductors are helically wound at a predetermined pitch with the conductors being laterally offset from one another and totally buried between the walls of the tube whereby many conductive signal paths can be established through the catheter without increasing its overall diameter. According to that disclosure, the inclusion of the helically wound conductors in the walls of the tube also allow the torque transfer, flexibility and structural properties to be tailored to fit a variety of applications. Such a catheter may be used as a cardiac pacer lead assembly or as an instrument for carrying out various diagnostic catheterization procedures.
It was in light of the foregoing that the present invention was conceived and has now been reduced to practice.